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Physics on a Swinging Tray: Greek Waiters Tray

Did you forget to pay the gravity bill this month? That's what people will think when they see your Greek Waiter's Tray defy gravity.

What you Need

Plastic plate or sturdy paper plate

6 feet or more of string

Plastic (unbreakable plastic) or paper cup

Water

What to Do

Poke three holes around the edge of the plate that are evenly spaced. A simple way to do this is to write the numbers 1 through 12 on the plate as if it were a clock face. Then poke holes through the numbers 12 o’clock, 4 o’clock and 8 o’clock. Be careful not to poke your hand that is holding the plate. (see figure 1)

Now we need to mark the center of the tray. For all practical purposes, this can be approximately by guessing. However to determine the center of a circle precisely as the ancient Greeks did with their trays, draw two chords connecting 12 to 4 and 12 to 8. Then draw a perpendicular bisector through each of these chords. This is a line drawn right through the middle of the chord and perpendicular to it. The intersection of these bisectors is the center of the plate. Mark the center with a marker or tape. (see figure 2)

Next, tie the end of the string to the 12 o’clock hole and loop it through the 4 o’clock hole and back up through the 8 o’clock hole. Pull the string up between the 12 and 4 so that the plate is suspended by three equal lengths of string. Twist and tie these pieces of string in a knot. (see figures 3 & 4)

Go outside and find a vast open region of space. Place the cup of water on the center of the plate and hold the knot with your fingers. Your tray is ready to defy gravity.

Finally, swing the tray side to side and confidently over your head. You must trust the physics when performing this stunt. Hesitation will get you wet. (see figures 5-7)

What’s Going On?

Often when you ride in a car, you feel as if something is pulling you toward the door during a turn. This is actually your body trying to continue forward while the car door pushes on you to go around the turn. The same thing is occurring with the tray. The cup of water wants to fly off the plate. But the plate presses on the bottom of the cup and keeps it moving in circular motion.

Try This!

Experiment with how fast and how slow you need to swing the tray around before the water spills. Try swinging the tray around your head like a helicopter.